In the field of data communications, units of data are communicated between devices having respective source and destination addresses in a high-speed communications network. Throughout the communications network, and between the devices, so-called “routers” are interconnected to, as their name suggests, route or forward the units of data from a communications device having the source address to a communications device having the destination address as they pass from router to router. In the communications network, the data communicated between the source address and the destination address can be subject to delays and hence can arrive at the destination address late. In some cases, the data may not arrive at the destination address at all.
It is therefore the task of a network diagnostics system to identify any latency in the communications network as a first step to curing or avoiding the cause of the latency. It is, of course, known to monitor traffic in the communications network for the purpose of supporting traffic engineering applications or accounting applications. In this respect, it is desirable to observe and characterise data units known as “packets”, in particular Internet Protocol (IP) packets, traversing the communications network.
One Service Assurance technology is known as Active Measurement Technology, and involves the generation, transmission and capture of well-formed synthetic traffic within a packet-switched network that supports, for example, Voice over IP (VoIP) calls to address a particular performance metric of interest in relation to a service. However, the measurements relate to the synthetic traffic and not real user traffic, and so do not reflect the experiences of the real user traffic.
An alternative technology is known as Passive Measurement Technology, and uses taps to couple first and second probes, respectively, to a link in the communications network at separate first and second respective points in the communicatons network in order to observe real user traffic on the link without disruption to any service being provided. These passive techniques rely on filtering, sampling and data reduction relating to observed real user traffic on the link with other annotations such as data capture timestamps. One example of such probes is the so-called Remote MONitoring (RMON) probe, as described in “SNMP, SNMPv2, SNMPv3, and RMON 1 and 2” (William Stallings, Addison Wesley). The RMON probe is used where a detailed view of network traffic is required, particularly when trouble-shooting.
U.S. Pat. No.5,521,907 describes a non-intrusive measurement apparatus that employs a first and a second probe respectively located at different points in the communications network for measuring delays across a link in the communications network. The probes are “programmed” with patterns to recognise, the patterns being present in packets that are the subject of a monitoring task, for example monitoring of a Session Initiation Protocol (SIP) call establishment dialogue. When the first probe recognises a pre-programmed pattern traversing the link at the point where the probe is located, the first probe generates a timestamp and a unique identifier for the packet bearing the pattern recognized. A similar procedure takes place at the second probe, if and when, the packet bearing the pre-programmed pattern is also recognised by the second probe. The timestamp and unique identifier pairs are respectively stored in buffers until the buffers are full, whereafter the collected data is forwarded to a measurement console, where the collected data is matched and used to generate round trip delays and travel times.
However, when packets are re-transmitted as is commonly the case in packet-switched communication networks, the above-described apparatus is not able to distinguish between a first transmission of a packet and a subsequent re-transmission, be it a first re-transmission of the packet or a fourth re-transmission of the packet. Consequently, incorrect travel and round-trip delay times can be calculated through use of timestamps indirectly assumed to signify receipt of an initial transmission of a packet. In any event, even if the probes are aware of re-transmissions of packets, no way is suggested of determining what packets each probe has recognised, and hence of identifying the data collected as relating to a particular re-transmission of a given packet.